1. Field of the Invention
The present invention generally relates to a spread spectrum communication system, and more particularly to a spread spectrum communication system in which a spread spectrum is obtained by switching of a plurality of pseudo noise codes (hereinafter referred to as PN code). In the spread spectrum communication system, it is difficult to intercept communication using delayed detection, so that tolerance to single code interference is improved. The spread spectrum communication system according to the present invention is applicable to indoor radio communication, weak radio communication, and mobile radio communication.
2. Description of the Related Art
There are two types of spread spectrum communication systems: a Direct Sequence (DS) system in which a signal is multiplied by a spreading code, such as a PN code, and a Frequency Hopping (FH) system in which a carrier frequency of a narrow band signal is switched at random and discretely. These two types of spread spectrum communication systems are disclosed in "Spread Spectrum communication system" (Mitsuo Yokoyama, Kagakugijutsu Publishing Company, 1998).
The spread spectrum communication has been known as a system having superior tolerance to interruption and superior secrecy. Recently, a CDMA (Code Division Multi Access) system has been particularly known.
The DS system which is one of the spread spectrum systems has an advantage in that the system can be simply formed. However, since only a single spreading code is used, it is easy to intercept communication using the delayed detection. In addition, the spread spectrum communication system has been popularized, but there is a limit to a number of spreading codes which can be in the spread spectrum communication Thus, a spreading code used in another terminal interferes with communication. This system has a problem of the single code interference.
As has been well known, in the FH (Frequency Hopping) system, hopping of a large number of frequencies is performed. Thus, in a high-speed hopping, this system has a superior tolerance to narrow band interference. By this invention, a spread spectrum code hopping (SS/CH) system in which a large number of codes are hopped is proposed. This system is expected to have a superior tolerance to the single code interference.
As has been described above, in the DS system, since only one spreading code is used, it is easy to intercept the communication using the delayed detection. Since there is a limit to a number of kinds of spreading codes, the DS system further has a problem of single code interference by other stations in the CDMA.
In the CDMA by the FH system, if a hopping frequency hits no (becomes equal to) that in another terminal, information cannot be transmitted or is damaged.